1. Field of the Invention
The present invention relates to a light emitting device and a manufacturing method thereof using a light emitting element which has a film containing an organic compound (hereinafter referred to as an “organic compound layer”) between a pair of electrodes and which can give fluorescence or luminescence by receiving an electric field. The light emitting device referred to in the present specification is an image display device, a light emitting device or a light source. Additionally, the following are included in examples of the light emitting device: a module wherein a connector, for example, a flexible printed circuit (FPC) or a tape automated bonding (TAB) tape, or a tape carrier package (TCP)) is set up onto a light emitting element; a module wherein a printed wiring board is set to the tip of a TAB tape or a TCP; and a module wherein integrated circuits (IC) are directly mounted on a light emitting element in a chip on glass (COG) manner.
2. Description of the Related Art
A light emitting element in the present invention is an element for emitting light by applying an electric field thereto. With respect to the light emitting mechanism, it is said that an electron injected from a cathode and a hole injected from an anode are recombined in an organic compound layer by applying a voltage to electrodes sandwiching an organic compound layer to produce a molecule with an excitation state (hereinafter referred to as “a molecular exciton”) and the molecular exciton releases energy to emit light when it is returned to a ground state.
In such a light emitting element, the organic compound layer is generally made from a thin film having a thickness less than 1 μm. In addition, since the light emitting element is a self-luminous type element such that the organic compound layer itself emits light, a back light used in a conventional liquid crystal display is not required. Thus, it is the big advantage that an extremely thin and lightweight light emitting element can be manufactured.
Also, when the carrier mobility of, for example, an organic compound layer having a thickness of about 100 nm to 200 nm is considered, a period from the injection of a carrier to the recombination is about several ten nanoseconds. Even when a period required for a process from the recombination of a carrier to light emission is included in the period, light emission is conducted within the order of microsecond. Thus, an extremely high response speed is one of characteristics thereof.
Further, since the light emitting element is a carrier injection type light emitting element, it can be driven by a direct current voltage and a noise is hard to generate. With respect to a drive voltage, the organic compound layer is made from a uniform ultra thin film having a thickness of about 100 nm, an electrode material such that a carrier injection barrier to the organic compound layer is decreased is selected, and a hetero structure (two-layers structure) is introduced. Thus, a sufficient luminance of 100 cd/m2 at 5.5 V has been achieved (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes” Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)).
From characteristics such as a thin type, lightweight, high speed responsibility, and direct-current low-voltage drive, the light emitting element has been noted as a next generation flat panel display element. In addition, since the light emitting element is a self-luminous type and has a wide viewing angle, the visibility is relatively good. Thus, it is considered that the light emitting element is effective as an element used for a display screen of an electronic apparatus.
There is a problem, however, with this type of light emitting element in that charge accumulates in an organic compound layer when using direct current drive, in which a unidirectional bias is always applied to the organic compound layer, thus reducing brightness.
It has been reported that reductions in brightness can be suppressed by inserting a hole injecting layer between an anode and a hole transporting layer, and in addition, by employing rectangular wave alternating current drive instead of the direct current drive (Reference 2: VanSlyke, S. A., Chen, C. H., and Tang, C. W., “Organic Electroluminescent Devices with Improved Stability”, Appl. Phys. Lett., 69, (15) 2160-2162 (1996)).
This is because there is an energy barrier relaxation due to the insertion of the hole injecting layer, and voltages having different polarities are applied alternately. The accumulation of charge in an inside portion of the organic compound layer is therefore relieved, and there is experimental support that reductions in brightness can thus be suppressed. There is also a suggestion that alternating current drive is suitable for increasing the element lifetime of light emitting elements.
However, light emitting elements driven by alternating current drive normally have a laminate structure made from an anode, an organic compound layer, and a cathode, and therefore electric current flows and light emission is obtained only when a positive voltage (forward bias) is applied from the anode side and a negative voltage (reverse bias) is applied from the cathode side. That is, if alternating current drive is used, the light emitting element will not emit light when a reverse bias is applied.
A brightness thus becomes dark if the effective display time is short, and therefore a problem develops in that deterioration of the light emitting element advances if a high voltage is applied so as to maintain a predetermined brightness.